1. Field of the Invention
This invention relates to a mechanism to maintain a constant body tube length of a binocular microscope, the body tube length being a distance from an objective lens or a ray dividing prism to an ocular lens of the binocular microscope.
2. Description of the Prior Art
A typical binocular microscope of the prior art has a ray dividing prism and two prisms receiving light rays from the ray dividing prism so as to refract the received light rays by 90.degree. toward two ocular lenses. If the pupillary distance between the two ocular lenses is modified by merely adjusting the spacing therebetween, the distance from the ray dividing prism to the ocular lenses is changed. Thus, the modification of the pupillary distance is inevitably accompanied with a change in the body tube length.
Various mechanisms to maintain a constant body tube length of the microscope are known. In a mechanism disclosed by the Japanese Pat. No. 8,187/1956, pins are secured to two ocular sleeves of a microscope, and the two pins are guided by two linear cam grooves slanted symmetrically on opposite sides with angles of 45.degree. relative to the optical axes of the ocular sleeves, so that when the pupillary distance is modified, the ocular sleeves are displaced in the axial directions thereof. This mechanism of the prior art is simple in construction but has a shortcoming in that backlashes are caused by gaps between the coacting linear cam grooves and the pins. It is apparent that when the backlash exists, accurate displacements of the ocular sleeves in the directions of the optical axes thereof in response to modification of the pupillary distance are impossible to achieve. Accordingly, the body tube length of the microscope cannot be maintained constant in the mechanism of the prior art.
To maintain the body tube length accurately constant is particularly important in microscopes of the type in which photographs are taken by focussing through adjustment of the ocular lenses of the body tube thereof. If there should be an error in the body tube length, even if focussing is done with the ocular lens of the body tube, the photographs may be blurred.